Changing DNS servers on xfinity gateway

We the users need to have an option to control our network. I don’t have to buy another modem just to change my dns server. We need the option manage our own network that we are paying for. Changing the dns on a device base is ridiculous, I don’t have time to change the dns settings on about 20 devices. And it reverts back to the xfinity settings after rebooting the device. Xfinity needs to give the customer more control, even the xfinity agent I spoke to couldn’t change it on their end. I am done with xfinity. I’m about to start searching For a new ISP.